ZnO/metal/ZnO (metal = Ag, Pt, Au) films for energy-saving in windows application

In this paper, the impact of different metals (Ag, Pt and Au) on the ZnO/metal/ZnO samples, being coated on a Glass substrate via RF/DC magnetron sputtering system, is investigated. The structural, optical, and thermal properties of the as-prepared samples were systematically studied for the purpose of storage and production of energy in industry. Our results show that these layers can be used as suitable coatings on building windows for energy storage applications. In the same experimental conditions, the case of Au as the intermediate layer has shown to have better optical and electrical conditions. Then the Pt layer also led to further improvement of the properties of the samples rather than those of the Ag. Moreover, the ZnO/Au/ZnO sample has shown the highest transmittance at the visible region (68.95%) and the highest FOM (5.1 × 10–4 Ω−1). Therefore, it can be considered as relatively the optimum sample in order for the building windows to save energy because of its low U-value (2.16 W/cm2 K) and low emissivity (0.45). Finally, by applying the equivalent voltage of 12 V at the ends of the sample, the surface temperature of sample has risen from 24 to 120 °C.

www.nature.com/scientificreports/ conductive electrodes, considering the Ag, Au, and Pt metals in the middle layer, in the same deposition conditions for achieving the optimum condition. Also, these multilayer electrodes are compared with single ZnO electrodes. For this purpose, FESEM and RBS analyzes are performed to evaluate the thickness and concentration of the elements of the samples. Besides, by measuring the UV-Vis-NIR spectrum of each sample, the energy gap and their optical properties are examined; and finally, using the Window7.8 simulation software, the thermal properties of each sample are studied. To evaluate and compare the as-prepared samples for use in industry, we calculated three important parameters; emissivity, figure of merit, and U-value. When the glass absorbs heat or light energy, it is either transferred to the outside by air current or reflected by the surface of the glass. In a material, the capability of radiating energy is known as emissivity. Mostly, fenestration products emit or radiate heat (long-wave far-infrared energy), which is of significance in energy saving. Therefore, improving their insulating properties would be possible by decreasing their heat emission. In this regard, the U-value has been suggested to be a good measure for the insulating quantity, which shows the rate of heat transfer in a fenestration product (W/m 2 °C). This parameter includes conductive, convective, and radiative heat transfer of a system. As one can deduce, the smaller the U-value of a material, the lower the rate of heat flow would be. In addition, by calculating the total R-value, the thermal resistance of a sample can be represented, which is the reciprocal of the total U-value (R = 1/U) 19 .

Experimental details
The ZnO sample was coated on a glass substrate by RF magnetron sputtering instrument (Nanostructured coatings co. DST3-T) deposition technique with the ZnO target (99.995% purity) with the RF power of 80 W. The substrate was situated at 75 mm distance from the target. ZnO deposition time in the top and bottom layers is 200 s with RF power. In the case of the middle layer, Au, Ag, and Pt metals with pure metal targets (99.995% purity) have been deposited at zero angles, and time deposition has been 40 s with DC power; the samples are nominated S 1 , S 2 , and S 3 , respectively.
Before each deposition, the chamber pressure reached 5 × 10 -3 Torr, and metals and ZnO target were presputtered for 5 min.
It is worth noting that the chamber has been flashed with Ar gas (99.995% purity) for three times to remove any contaminants. After preparing the samples, the four-point probe technique and Ultraviolet-Visible-Near IR (UV-Vis-NIR) spectrophotometer (JASCO V-670) were used to determine sheet resistance and optical properties of the samples, respectively.
To evaluate the elemental concentration and thickness of each layer, Rutherford backscattering spectrometry (RBS) analysis has been provided using a helium beam with 2 MeV energy. The experiment was performed on a 15° left lane line of Tehran Vandograph Laboratory in a conventional pixie chamber. A surface barrier detector at the angle of 165° was used to record the scattered particles. The reported data error related to fitting, cross-sectional data, beam energy, and incident load measurement is found to be 10%. The multilayer films' total thickness was determined with field emission scanning electron microscope (FESEM) cross-section (Zeiss Sigma 300-HV).

Results and discussion
Rutherford backscattering analysis. Figure 1 depicts 2 MeV RBS spectra of ZnO/metal/ZnO samples for different metals (Ag, Au, Pt). According to the shape of the diagrams simulated by SIMNRA software, they are well fitted with experimental data. As a result, the thickness and concentration of the material can be appropriately determined. These curves provide the backscattered energies of the incident particles for all samples. The channel region at 500-600 and 600-700 nm, respectively, contains a peak related to Zn, Au, Ag, and Pt metals. The valley's depth between the Zn-Au and Zn-Pt signals is equal, and the valley between the Zn-Ag signals is reduced, while interlayer penetration is increased 20 . The thicknesses have been calculated in terms of monolayer (10 15 atoms/cm 2 ), corresponding to an areal atomic density and assuming uniform layer distribution. In the case that we consider the nominal stoichiometry of known atomic density (5.9 × 10 22 atoms/cm 3 for Au, 5.8 × 10 22 atoms/cm 3 for Ag, and 6.6 × 10 22 atoms/cm 3 for Pt), the thickness can be readily obtained in nm scale. The areal density of the Au, Ag, and Pt metals at the middle layer obtained from the RBS analysis are 25.5, 32.2 and 27.31 (× 10 15 atoms/cm 2 ), respectively. As a result, assuming the layer is uniform, the thickness of the Au, Ag, and Pt metal layers in the S 1 , S 2 and S 3 samples is are 4.08, 5.44 and 4.09 nm, respectively 21 .
The FESEM cross-section. Figure 2 shows the cross-section FESEM images of all samples. In the same experimental conditions, S 1 , S 2 , and S 3 samples' thicknesses are found to be 77, 61 and 63 nm, respectively.
Calculation of sheet resistance. The sheet resistance of all the samples are measured using a four-point probe, which are shown in Table 1. The sheet resistance of the samples in the same conditions and duration of the coating indicates that the resistance of the ZnO single layer sample is very high. The presence of the middle layer of the metal reduces the electrical resistance. As shown in its strip structure, these structures can be considered connecting metal to a semiconductor. In the study of the structure of ZnO/metal, no barrier for transferring the electrons from the metal to the semiconductor is found after bonding; the electrons are readily transferred from the metal layer to the ZnO and conversely. In this case, the density of charge carriers is increased due to the injection of electrons from the metal into the semiconductor or conversely 20,22 . The sheet resistance of sample with Ag metal is higher than those of the samples coated with Au and Pt metals; while in most articles on three-layer structures, Ag metal was used in the middle layer, and low resistance is mentioned. Ag grows in Volmer-Weber (island) mode on oxide substrates. The high resistance in the sample with Ag metal is probably as a result of the presence of separate islands on the surface of Ag metal, and for the disappearance and cohesion of these islands, more Ag coating layer is required. According to the results obtained in our previous article 23  www.nature.com/scientificreports/ below 10 nm, Ag is deposited as a discontinuous island. This indicates that having a three-layer structure with different metals requires more Ag than Au and Pt, which is more cost-effective.
Calculation of transmittance, reflectance, and average transmittance. The transmission and reflection spectra of samples have been provided in the wavelengths of 190-2700 nm, shown in Fig. 3. In Fig. 3a,   Table 2.
The transmission coefficient is obtained in the following: where V(λ) and T(λ) are, respectively, the luminous spectral efficiency and the transmission of the samples 19,23 .
The T vis values of the S 1 , S 2 , and S 3 samples are 68.95, 58.76 and 47.54%, respectively; the highest value belongs to the S 1 sample.
The comparison of the reflection in the IR region for three-layer ZnO/metal/ZnO electrodes considering different metals in the same fabrication process has not been investigated. Infrared reflectance is one of the most crucial parameters of electrodes for use in industry. In this study, the comparison of the reflectance of these electrodes in the IR region was investigated for the first time.
According to Fig. 3b, the reflection of the S 1 , S 2 , and S 3 samples in the near-infrared region is equal to 19, 35 and 36% at a wavelength of 1700 nm, respectively. ZnO single layer has the lowest reflectance in this range. Table 2 shows the average reflection in the solar and near-infrared regions, which is calculated using Eq. (1), with   Calculation of FOM parameter. Both conductivity and transparency parameters are very critical in the industry. For a better comparison of the properties of metal-oxide/metal/metal-oxide three-layer electrodes, the figure of merit parameter is used, which can be calculated from the following equation 24 .
The larger the amount of FOM, the more transparent the conductive electrode will be. The FOM values of the samples are shown in Table 3. According to the mentioned values, the maximum amount of FOM is related to the S 1 sample with the value of 5.1 × 10 -4 Ω −1 .
Calculation of emissivity. Transparent conductive electrodes with low emissivity characteristics are a razor-thin, colorless, non-toxic coating that are used in window glass to enhance energy efficiency. Such windows are remarkably safe and are becoming standardized from the energy efficiency point of view in the modern home. Low-E windows prevent infrared light from penetrating into the glass from the outside. Moreover, these windows keep heating/cooling energy. The emissivity is dependent of the R sh and is obtainable in the wavelength range of 780-2700 nm using the following equation 6,19,20 Besides that, for samples having the equivalent wavelengths of λ > 3 µm and R sh ˂˂ Z 0 are also obtainable as follow: where Z 0 is the impedance of the vacuum (377 Ω). The acquired data of emissivity for all the samples are shown in Table 3. The minimum amount of emissivity is related to the S 1 sample with the value of 0.45.

Calculation of bandgap energy (E g ).
The absorption coefficient (α) has been obtained for the direct transition using the following equation 25 : where h and A represent the incident radiation energy and a constant, respectively. The direct E g is acquired by extrapolating the linear parts of the plots to zero absorption (αhυ = 0). The E g changes for the ZnO/metal/ZnO multilayer system is provided in Fig. 4. According to the results obtained from the calculation of the bandgap energy, the energy gap of the ZnO sample is equal to 3.31 eV, which decreases for the ZnO/M/ZnO (M = Au, Ag, and Pt) thin-film samples. These bandgap changes in three-layer structures with different metals are consistent with the transmission changes in the visible region, so that the transmittance of ZnO single layer was maximum, and it can be seen that the band gap for this sample is also maximum. Also, the transmittance of the S 1 sample is higher than that of the S 2 sample, and the bandgap of the S 3 sample is minimal 26 . Heating results. For studying the heating impacts of samples, their electro-thermal behavior is examined and shown in Fig. 5. For this purpose, silver contacts have been coated on both sides of the samples by an electron beam evaporator. Then, by applying a certain voltage for 300 s, the maximum temperature of produced heat between the contacts was measured by a thermal camera. According to Fig. 5, the temperature of the S 1 sample, with a voltage change from 4 to 12 V, has a sharp increase in temperature from 35 to 120 °C, while this temperature increment is found to be less for the S 3 sample (from 30 to 80 °C), and the S 2 sample did not show any temperature change with increasing voltage.
Here, the heat loss as a result of the conduction and radiation from the back can be neglected since the glass substrate is not a good thermal conductor. Therefore, the prominent path of heat loss, which is air convection, is obtainable according to the following formula 27 : www.nature.com/scientificreports/ Q g is heat generated at power P for a time duration of Δt, h conv is the convective heat transfer coefficient, A conv is the surface area, and t s and t i are the saturation and initial temperatures, respectively. As one can infer, with the voltage increment and the resistance decrement, the saturation temperature increases. The S 1 sample has less sheet resistance and more h conv than the S 3 sample, and for this reason, it can be seen in Fig. 5 that the amount of heat production in the S 1 sample at a specific voltage is significantly increased compared to that of the S 3 sample. The temperature of the S 2 sample, probably due to its high resistance, did not decrease below 4 to 12 V. Thermal performance. To estimate the amount of heat passing through the material, the T vis , R vis , T solar , R solar , T NIR , R NIR and emissivity values of the samples must be calculated 28 . The T vis and R vis are the rate of transmission and reflection in the area of 400 < λ < 800 nm, T solar , R solar are the rate of transmittance and reflection in the area of 250 < λ < 2500 nm, and T NIR , R NIR are the rate of transmittance and reflection in the area of 780 < λ < 2500 nm; all of which are shown in Table 2. In this study, to calculate the U-value of each sample, using Window7.8 software, we simulated a double-glazed system comprised of two layers of glass with a thickness of 4 mm and a gap layer containing argon gas. The U-values of all samples are listed in Table 3. In the absence of the ZnO/metal/ZnO coating on double-glazed windows, the U-value is 2.730 W/m 2 K. However, after the deposition process of the transparent conductive electrode coating, the U-value decreased significantly. The S 1 (ZnO/ Au/ZnO) sample has the lowest, and the S 2 (ZnO/Ag/ZnO) sample has the highest U-value due to its high sheet resistance and high emissivity.

Conclusion
In this work, the structural, optical, electrical, and thermal properties of ZnO/metal/ZnO three-layer films are studied, in which Au, Ag, and Pt metals are deposited in the middle layer on glass substrate using the magnetron sputtering technique. The structural properties of the samples have been studied using RBS and FESEM analyzes. In addition, the optical properties of the material were investigated by measuring their transmission and reflection spectrum in the range of 190-2700 nm. The lowest amount of resistance before and after annealing is related to the ZnO/Au/ZnO sample, followed by the highest amount of heat production by applying voltage. The transmittance and reflectance spectra of the samples show that the highest values of the average transmittance in the visible region and the reflectance at the wavelength of 1700 nm are related to the ZnO/Au/ZnO and ZnO/Pt/ZnO samples, respectively. Also, the FOM values in the ZnO/Au/ZnO sample has a maximum value of 5.1 × 10 -4 Ω −1 . The FOM values of ZnO/Ag/ZnO and ZnO/Pt/ZnO samples are 0.136 × 10 -4 and 0.105 × 10 -4 (Ω −1 ), respectively. The amounts of emissivity, T av , and R 1700 are very effective in heat transfer rate through the material (U-value). For example, the ZnO/Au/ZnO sample has the lowest U-value of 2.16 W/m 2 K. According to the provided analyzes and calculations, ZnO/Au/ZnO and ZnO/Pt/ZnO samples have a better performance in heat production by applying voltage and reducing the transfer of heat through matter and emissivity than ZnO/Ag/ZnO.

Data availability
The datasets used and analyzed during the current study available from the corresponding author on reasonable request.